1. Field of the Invention
The present invention relates to an active-matrix liquid crystal display (LCD) device, and to a method of fabricating the same. More particularly it relates to an array substrate for an active-matrix LCD device having thin film transistors, and to a method of fabricating that array substrate.
2. Discussion of the Related Art
An active matrix type LCD device usually uses thin film transistors (TFTs) as switching devices. An LCD device is typically made up of two substrates having an interposed liquid crystal material. One substrate, referred to as the array substrate, includes a matrix array of TFTs and pixel electrodes. The opposing substrate, referred to as the color filter substrate, includes a light-shielding film (also known as a black matrix), a color filter, and a common electrode.
Because of its simple structure and superior quality, an inverted staggered type TFT is widely used on array substrates. The inverted, staggered type TFT can be classified into either a back-channel-etch type or an etch-stopper type. Those types are differentiated according to the methods of forming a channel in the TFT. Of the two, the back-channel-etch type has a simpler structure.
A typical array substrate manufacturing process requires repeated steps of depositing and patterning of various layers. The patterning steps use photolithography masks. Each step is facilitated by using one mask. The number of masks used in the manufacturing process is a critical factor in determining the number of patterning steps. In particular, manufacturing costs depend heavily on the number of masks used. Furthermore, the reliability of the resulting device can depend upon the number of patterning steps used.
Referring to the attached drawings, an array substrate of an LCD device that incorporates a back-channel-etching type TFT structure and that is manufactured by a conventional method will now be explained in some detail.
As shown in FIG. 1, the LCD device 20 includes an array substrate 2, a color filter substrate 4 opposing the array substrate 2, an interposed liquid crystal 10, and a sealant 6 that is formed at the periphery of the gap between the two substrates 2 and 4. The sealant 6 prevents the liquid crystal 10 from leaking out of the LCD device 20.
The array substrate 2 includes a substrate 1, a TFT 5, and a pixel electrode 14. The TFT 5 acts as a switching element for changing the orientation of the liquid crystal 10, and the pixel electrode 14 is used as a first electrode to apply electric fields across the liquid crystal 10.
The color filter substrate 4 includes a substrate 11, a color filter 8, and a common electrode 12. The color filter 8 is used for displaying colors and the common electrode 12 is used as a second electrode to apply electric fields across the liquid crystal 10.
Referring to FIG. 2, a more detailed description of the structure and operation of the array substrate 2 will be provided.
On the substrate 1, a gate line 22 is formed in a horizontal direction and a data line 24 is formed in a transverse direction. The pixel electrode 14 is formed within a rectangular area partially defined by the gate and data lines 22 and 24. Sometimes the pixel electrode 14 will overlap the gate and date lines 22 and 24. Near the crossing point of the gate and data lines 22 and 24, a portion of the gate line 22 forms a gate electrode 26. At one end of the gate line 22 is a gate pad 18 having a gate pad contact hole 21.
Near the crossing point of the gate and data lines 22 and 24, the data line 24 protrudes to form a source electrode 28. A drain electrode 30 is then formed at a position that is spaced apart from the source electrode 28. At one end of the data line 24 is a data pad 20 having a data pad contact hole 23.
Spaced apart from the drain electrode 30 and over a portion of the gate line 22 is an island-shaped capacitor electrode 32 that is formed at the same layer as the data line 24. A protruding portion of the pixel electrode 14 overlaps the capacitor electrode 32, and together with the gate line 22, forms a storage capacitor 7 that stores electric charges.
A capacitor contact hole 36 enables the capacitor electrode 32 to electrically connect to the pixel electrode 14. Another portion of the pixel electrode 14 overlaps a portion of the drain electrode 30. A drain contact hole 34 at the overlapped portion enables the pixel electrode 14 to electrically connect to the drain electrode 30.
As explained previously, the TFT 5, which includes the gate, source, and drain electrodes 26, 28 and 30, selectively applies an electric field to the liquid crystal 10 (shown in FIG. 1). In operation, if a signal is applied to the gate electrode 26 of the TFT 5, an electrical connection is established between the data line 24 and the pixel electrode 14. With the gate electrode 26 turned ON, an electric field is produced by the pixel electrode 14 in accordance with the signal applied to the data line 24 via the data pad 20.
Next, referring to FIGS. 3A to 7A and 3B to 7B, a more detailed description of the structure and the fabrication method of the TFT and the storage capacitor will be provided. FIGS. 3A to 7A illustrate sequential fabrication steps of a cross-section taken along a line “A-A” of FIG. 2, and FIGS. 3B to 7B illustrate corresponding sequential fabrication steps of a cross-section taken along a line “B-B” of FIG. 2.
As shown in FIGS. 3A and 3B, a first metallic material is deposited on a surface of the substrate 1. That metallic material is then patterned using a first mask to form the gate line 22, including the gate electrode 26. Also formed at this time is the gate pad 18 shown in FIG. 2. For the first metallic material, a highly conductive metal such as aluminum (Al), aluminum alloy, or molybdenum (Mo) is preferred.
As shown in FIGS. 4A and 4B, a first insulating material is then deposited to form a gate insulating layer 50. On the gate insulating layer 50 a semiconductor material is then deposited and doped with impurities. That semiconductor material is then patterned with a second mask to form a semiconductor layer 52 having an ohmic contact layer 54. This defines a first intermediate structure.
Then, as shown in FIGS. 5A and 5B, a second metallic material is deposited over the first intermediate structure and patterned using a third mask to form a source electrode 28, a drain electrode 30, and a data line 24. The data line 24 is connected to the source electrode 28 (FIG. 5A). At the same time, over a portion of the gate line 22, the second metallic material is used to form a capacitor electrode 32 while using the third mask (FIG. 5B).
Afterwards, a portion of the ohmic contact layer 54 is etched away to define a channel region 56 on the semiconductor layer 52 (FIG. 5A). At this point a second intermediate structure is defined. That second intermediate structure includes the TFT 5 comprised of the gate, source, and drain electrodes 26, 28, and 30, the semiconductor layer 52 having the channel region 56, and the ohmic contact layer 54.
As shown in FIGS. 6A and 6B, a second insulating material is deposited over the second intermediate structure. That second insulating material is then patterned using a fourth mask to form a passivation layer 58. The passivation layer 58, which protects the TFT 5 and the capacitor electrode 32, is beneficially comprised of inorganic-based silicon nitride (SiNx), of silicon oxide (SiO2), or of an organic-based benzocyclobutene (BCB). Those materials are beneficial because they exhibit high light-transmissivity, are relatively moisture-proof, and have high durability. Patterning the second insulating layer using the fourth mask also forms a data pad contact 23, a drain contact 34, and a capacitor contact hole 36. The result is a third intermediate structure.
Then, as shown in FIGS. 7A and 7B, a transparent conductive material is deposited on the third intermediate structure. That transparent conductive material is then patterned using a fifth mask to form the pixel electrode 14. The pixel electrode 14 is electrically connected to the drain electrode 30 and to the capacitor electrode 32 via the drain and capacitor contact holes 34 and 36, respectively. The transparent conductive material is beneficially made of indium tin oxide (ITO).
FIG. 8 shows the above-described fabricating process in a block diagram.
In step ST200, the substrate is cleaned to be free from surface contaminants.
In step ST210, the gate line 22, gate electrode 26, and gate pad 18 are formed by depositing the first metallic material and patterning the first metal layer using the first mask.
In step ST220, the gate insulating layer 50 is deposited and patterned using the second mask. Then, the semiconductor 52 and the ohmic contact layer 54 are formed by sequentially depositing and patterning a semiconductor layer and a doped semiconductor layer. The ohmic contact layer 54 can be formed by doping impurity ions into the semiconductor layer instead of by depositing a doped semiconductor layer.
In step ST230, the source and drain electrodes 28 and 30, the data line 24, and the capacitor electrode 32 are formed by depositing and patterning the second metallic layer using a third mask.
In step ST240, the back channel 56 is formed by etching the ohmic contact layer 54 using the source and drain electrodes as a mask.
In step ST250, a second insulating layer is deposited and the passivation layer 58, the data, the drain, and the capacitor contact holes (23, 34, and 36 respectively) are formed by patterning using a fourth mask.
In step ST260, the pixel electrode 14 is formed by depositing and patterning the transparent conductive material using a fifth mask.
The above-described conventional method of fabricating the array substrate of the LCD device employs five masks. If aluminum is used to form the gate electrode, at least two additional masks are needed to prevent hillocks that could lead to gate line defects.
A more detailed description of the hillock problem is provided with references to FIG. 9, which is an enlarged view of the storage capacitor 7 of FIG. 6B. A hillock “H” occurring on the surface of the aluminum gate electrode 22 can impact growth of the gate insulating layer 50 on the gate electrode 22. An abnormally grown insulating layer may induce a short between the gate electrode 22 and the capacitor electrode 32, thus deteriorating the display characteristics. Additional mask processes are needed for oxidizing the gate electrode 22 so as to prevent hillocks. Accordingly, at least five, and as many as seven, masking steps, are required in the conventional fabricating process of the array substrate.
As indicated above, a decrease in the number of masking steps would decrease the manufacturing costs. Furthermore, a decrease in the number of masking steps can improve the manufacturing yield.